Tubes and pipes are naturally quite inaccessible with regard to their inner wall surfaces. The surfaces are usually coated by immersing the tube into a liquid bath and rotating the tube. These procedures are not available when the tube is integrated in a heat exchanger array or the like. Condensers in power plants, for instance, often have tube arrays with hundreds of parallel and closely spaced pipes. These pipes are accessible only from the header or footer plates.
Condenser tubes are subject to extreme heat differentials, to corrosive media, and to solids deposits. During service, the tube walls are slowly abraded or corroded and subject to deposits forming thereon. The inner wall surfaces must therefore be cleaned from time to time and the protective coating on the inner wall surface must be refurnished to prevent further corrosion or abrasion and to diminish fouling.